The present invention relates to a method and system for controlling the idle speed of a spark ignition internal combustion engine to suppress the variation of idle engine speed under varying operating conditions.
Known systems for controlling the idle speed of an internal combustion engine involve the use of an engine's intake air or air-fuel ratio as a controlling factor to suppress the tendency of the idle speed to vary significantly in response to a rapid change in engine load. In the known systems an auxiliary air delivery system or fuel control system is provided to control the amount of such fluidic substances to counteract the idle speed variation.
However, due to the slow response characteristics inherent in the air and fuel control systems the known idle speed control systems fail to provide effective additional output torque until the engine speed reduces considerably to a low level and are thus not satisfactory to counteract rapid changes in engine load. Furthermore, if the air-fuel ratio of the engine is controlled in a closed loop for the purpose of reducing noxious emissions, an excessive amount of the controlling parameter of the known idle speed control system would interfere with the closed-loop controlled air-fuel ratio, resulting in the latter hunting or oscillating above and below an optimum value.